This application claims priority to German Application 101 14 415.6, filed on Mar. 23, 2001, the entire teachings of which are incorporated herein by reference.
An injection molding process used in the manufacture of molded parts from thermoplastic is known, for example, from U.S. Pat. No. 4,101,617. A fluid under pressure is introduced into the still molten plastic melt. The melt is pressed against the cavity wall of the injection-molding die by the pressure thus produced in the interior of the melt; collapsed points due to contraction of volume during cooling are thus avoided.
Nitrogen gas is conventionally used as the fluid, which is injected into the melt under pressure. It has the advantage that as an inert gas, it is not subjected to a chemical reaction in the hot melt. The disadvantage is thus taken into account that the nitrogen is in most cases very expensive to produce. Either the injection-molding device has to be supplied with nitrogen from cylinders or with liquid nitrogen or the gas is recovered on site xe2x80x94for greater gas requirementxe2x80x94for example, by means of molecular filters.
The requirements increasing ever further on the economic viability of the process necessitate ever shorter injection-molding cycles with as high as possible quality of the molded parts to be manufactured. In order to achieve the shorter cycles, various attachments have been made:
European granted patent 0 400 308 proposes, after gas injection into the melt, to allow the gas to emerge again at a point remote from the point of injection; circulation of the gas through the cavity provided is then effected. A cooler is integrated into the gas circuit. A more rapid cooling process for the melt should thus be effected, since cooled gas is supplied in the closed circuit.
German Offenlegungsschrift 4 219 915 however is geared to using cooled gas, which is injected into the melt. Provision is thus made, namely, in that the gas is cooled to temperatures as far as xe2x88x92160xc2x0 C. The plastic material should cool as rapidly as possible in this manner; the time span from injection of the melt into the injection-molding die to releasing is thus reduced.
In the previously known processes, it is disadvantageous that, firstly now as before, expensive nitrogen is necessary, and secondly in spite of all measures, the cooling effect remains limited due to the restricted thermal capacity of the gas. Regardless of that, due to the considerable difference in viscosity between melt and gas, there is the problem that flow markings appear now and then on the surface of the molded part, which negatively influence the quality of the molded part to be manufactured.
One aspect of the invention is therefore to further develop the injection-molding process of the generic type, so that the said disadvantages are avoided at least in part. The process should thus make it possible to manage without the use of expensive nitrogen. Furthermore, as short as possible a cooling time should be realizable, which noticeably shortens the injection-molding process. Finally, the process should also ensure that as homogeneous as possible a flow path of the melt into the injection-molding die takes place, so that transfer markings can be avoided as far as possible.
In one embodiment, a process is provided for injection molding of a molded part made from thermoplastic plastic material having at least one cavity. This process includes the injection of plastic melt from an injection unit along a melt flow path into a cavity of an injection-molding die to completely volumetrically fill the cavity with plastic melt in step a). Liquid is injected in step b) into the still molten plastic material so that the latter is pressed against walls of the cavity to form the at least one cavity in the molded part. Compressed gas is introduced into the at least one cavity formed by the liquid. The plastic material is then allowed to cool in step c) until the latter forms the molded parts in a self-supporting manner. The molded part is then released from the cavity of the injection molding die in step d).
A concept of the invention is thus geared to using a liquid having high thermal capacity as fluid to be injected into the melt, wherein this ensures that a rapid cooling process takes place, so that the cycle time of the injection-molding cycle can be noticeably reduced. The additional addition of a compressed gas, for example air, nitrogen (N2) or carbon dioxide (CO2) can be used here, supports the molded-part forming process. A high pressure can be maintained by the gas until the molded part is self-supporting. Furthermore, it is possible to introduce the liquid so that as far as possible, a channel for the gas is provided and this then ensures the actual molded-part formation. This means consequently, that in the first step liquid is added in order to pre-shape a cavity, then gas is introduced in order to at least partly remove the liquid and then the gas is held under pressure in order to form the molded part.
Any group of thermoplastics, regardless of whether they are provided with additives, such as glass fibers, chemical or physical propellants or similar, is provided here as thermoplastic plastic material.
During this procedure, it has been shown, surprisingly, that due to the said features, flow markings hardly appear, which are otherwise to be feared and to be observed during the gas-internal pressure process. This is attributed to the comparable viscosity of the liquid melt with the injected liquid of high thermal capacity.
According to a first further development, provision is made in that during the above process step b), some of the still molten plastic material is displaced from the cavity into a spillover cavity. The flow of plastic material from the cavity into the side cavity is thus controlled by valve means in one embodiment, which are opened or closed according to a temporal model. Specific influencing of the overflow of melt from the main to the side cavity is thus possible. Furthermore, it is conceivable to use several side cavities, which are controlled independently of one another.
The liquid can be injected into the cavity along the melt flow path through the sprue region directly or via the machine nozzle, through which the plastic melt is supplied, or alternatively to that into the cavity directly by means of an injection nozzle, wherein when the requirement is to form several cavities, a separate injection nozzle is provided for each cavity.
In the second case, it can be advantageous if some of the plastic material situated in the cavity is driven back out again from the cavity during the above step b) by the injected liquid in the direction of the injection unit.
A further improvement in the flow behavior of the melt or the control of this behavior can be seen when, before injection of the thermoplastic plastic melt, a pressure which is increased with respect to the ambient pressure is built up in the cavity by introducing a gas. This gas pressure can be controlled and/or regulated during the above step a) as a function of how the injection pressure of the melt increases during its injection. In one embodiment, the gas pressure is operated according to a predetermined pressure or time profile.
It has proved to be a particularly advantageous embodiment that the liquid is tempered before injection into the still molten plastic material. The thought here is namely that the liquid is cooled to a preset temperature range. A temperature range between about xe2x88x9220xc2x0 C. and +20xc2x0 C., in one embodiment between about 4xc2x0 C. and 15xc2x0 C., is thus provided particularly advantageously. However, it can also be necessary, for example for materials which are damaged by sudden cooling, to heat the liquid to a preset temperature range. A temperature range between about 20xc2x0 C. and 150xc2x0 C., in one embodiment between about 40xc2x0 C. and 100xc2x0 C., is thus provided particularly advantageously. At the temperature ranges indicated above, the use of water was thought of first and foremost. However, it is also conceivable to use, for example liquefied gases, such as carbon dioxide or nitrogen, in order to particularly strengthen the cooling effect. When using such liquids, a temperature range between about xe2x88x92150xc2x0 C. and xe2x88x9220xc2x0 C. is offered, wherein the range from about xe2x88x9260xc2x0 C. to xe2x88x9240xc2x0 C. is used in a particular embodiment. Since the liquid is added to the melt under increased pressure, it ensures that the boiling temperature is increased and hence the fluid can be added in liquid form.
In one concept of the invention, particular significance is given to the removal of the injected liquidxe2x80x94possibly still in the injection-molding device. There are several possibilities here in accordance with one embodiment of the invention.
First of all, provision can be made in that after allowing to cool and before releasing, the following process step is executed:
cxe2x80x2) injection of compressed gas, in one embodiment compressed air, along the path, through which the liquid was injected into the plastic material, and blowing-off of the liquid from the cavity of the molded part at at least one blow-off point, which is situated at a point, which is remote from the addition point of the liquid.
The blow-off point, in one embodiment, is arranged in the region of the flow path end of the plastic material. In order to achieve a further increased cooling effect, liquid can be added again according to step cxe2x80x2) instead of compressed gas and hence circulation of the liquid through the molded part can be achieved.
Alternatively to that, provision can be made in that at the said point in time, the following process step is executed:
cxe2x80x3) injection of compressed gas, in one embodiment, compressed air, at a gas addition point which is remote from the point at which the liquid was injected into the plastic material, and blowing-off of the liquid from the cavity of the molded part via the point at which the liquid was injected into the plastic material.
Provision can thus be made to use means which add and discharge again the liquid and/or the compressed gas via the same media path via a suitable transfer device.
Apart from blowing-off of the liquid from the cavity, drawing-off of the liquid is also suitable. Then provision is made in that the following process step is executed:
cxe2x80x2xe2x80x3) applying a vacuum at a point fluidly connected to the liquid-filled cavity, in order to draw off the liquid situated in the cavity from the cavity.
The vacuum is thus applied to the injection nozzle for liquid in an advantageous manner. In order to ensure that the molded part is not drawn in on itself by the vacuum, a ventilation opening can be provided before or during application of the vacuum.
The molded-part forming process during injection of melt and of liquid and during curing of the material, can be favorably influenced by appropriate control or regulation of the media pressures. Provision is therefore advantageously made in that a pressure, which is regulated or controlled according to a preset time profile, is exerted on the liquid introduced into the plastic melt during the above steps b) and/or c). The pressure profile can thus be regulated or controlled according to a pulsating rising and falling path; above all the thought is thus of a pressure path running sinusoidally about a preset constant pressure value.
Alternatively, provision can also be made in that the pressure in a first time phase is regulated or controlled at a first, in one embodiment constant, level and in a later, second time phase at a second, in one embodiment constant, level, wherein the first pressure level is lower than the second. The pressure can thus act both on the added liquid and on the additionally added compressed gas, since the compressed gas can also be added during the addition of the liquid.
As a farther advantageous further development, the injection-molding die can be designed so that the cavity or the volume of the cavity forming the molded part is increased before, during or after step b), that is addition of the fluid into the still molten plastic material. This embodiment of an injection-molding die is generally termed as a breathing die.
After the liquid has been removed from the cavity, the opening(s) to the cavity can be sealed by re-injection of melt.
Water, oil or alcohol is suitable as the liquid having high thermal capacity. However, it is also conceivable to use liquid carbon dioxide or liquid nitrogen.